feat(tvix/eval): implement OpForce in VM

This operation forces the evaluation of a thunk.

There is some potential here for making an implementation that avoids
some copies, but the thunk machinery is tricky to get right so the
first priority is to make sure it is correct by keeping the
implementation simple.

Change-Id: Ib381455b02f42ded717faff63f55afed4c8fb7e3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6352
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/opcode.rs

@@ -109,6 +109,7 @@ pub enum OpCode {
 
     // Thunks
     OpThunk(ConstantIdx),
+    OpForce,
 
     /// Finalise initialisation of the upvalues of the value in the
     /// given stack index after the scope is fully bound.